Autosomal recessive generalized myotonia

Novel chloride channel gene mutations in two unrelated Japanese families with Becker's autosomal recessive generalized myotonia

We investigated the skeletal muscle voltage-gated chloride channel gene (CLCN1) in two unrelated Japanese patients with Becker's myotonia congenita. The non-myotonic parents of each patient were consanguineous. The proband of each family shares generalized myotonia, transient weakness after rest, and leg muscle hypertrophy. However, the disease severity related to the degree of myotonia differed, even in view of the response to long train nerve stimulation tests. CLCN1 gene analysis revealed a novel Ala659Val missense mutation identified to be homozygous in the more severe patient, while a novel Gln445Stop nonsense mutation was present in the other patient. Both mutations were absent in 90 Japanese normal controls. This is the first report of Japanese cases of Becker's myotonia congenita with CLCN1 gene mutations.

Investigation of muscle disease

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Evidence for genetic homogeneity in autosomal recessive generalised myotonia (Becker)

Generalised myotonia Becker (GM) is an autosomal recessively inherited muscle disorder. Affected subjects exhibit myotonic muscle stiffness in all skeletal muscles with marked hypertrophy in the legs. A transient muscle weakness is particularly pronounced in the arms and hands and is a typical symptom of the disorder. Recently, we showed complete linkage of the disorder GM to the gene (CLCN1) coding for the skeletal muscle chloride channel CLC-1 and the TCRB gene on chromosome 7 in German families. In the study presented here we performed linkage analysis on 14 new GM families. The GM locus was again completely linked to both the CLCN1 and the TCRB gene in all families with a combined lod score of Z = 9.26 at a recombination fraction of theta = 0.00. This confirms our previous data and supports the hypothesis that GM is a genetically homogeneous disorder. The previously detected T to G missense mutation is found on 15% of the 66 GM chromosomes counted so far.

Case-of-the-month: autosomal recessive myotonia congenita: marked muscle weakness in a 16-year-old boy

A 16-year-old boy had a 10-year history of stiffness in leg muscles. There was marked weakness of neck flexors, shoulder abductors, and ankle dorsiflexors, with hypertrophy of most muscle groups and both action and percussion myotonia. The parents were normal. Motor unit potential mean duration was reduced in the weakest muscle (tibialis anterior), and a biopsy of the same muscle showed only minimal abnormalities. Exercise and repetitive stimulation (30 Hz) of the tibialis anterior disclosed a decline in the compound muscle action potential that appeared to correlate with the muscular weakness.

AAEE minimonograph #27: differential diagnosis of myotonic syndromes

Recent advances in neuromuscular diseases have also widened the diagnostic spectrum of myotonic disorders. Treatment, prognosis, and genetic aspects are different in the various syndromes and mandate a correct diagnosis. The combination of neurologic examination, standard EMG, exercise test, cold exposure, potassium loading, eye examination, and pedigree analysis allows correct classification of nearly all patients with myotonic disorders. In this review emphasis is placed on clinical features and electrophysiologic evaluation.

Successful treatment with tocainide of recessive generalized congenital myotonia

A patient with recessive generalized congenital myotonia and severe, disabling weakness underwent various forms of treatment while being monitored electrophysiologically. Phenytoin, verapamil, and acetazolamide were ineffective, but tocainide yielded good results. Improvement was dose-dependent, and was limited by irritability and action tremor when the patient was taking 1,600 mg per day.

Repetitive nerve stimulation in the differential diagnosis of congenital myotonia

Repetitive stimulation of the ulnar nerve at 5 Hz was performed in 9 patients with congenital myotonia, 3 with dominant and 6 with recessive transmission. At this frequency the motor response differed markedly, all the patients with the recessive form presenting a clear decremental response to repeated stimulation. The size and time course of the decrease seem to be related to the degree of muscular paresis. The series includes one case of dominant congenital myotonia type II, whose response to repetitive stimulation, reported for the first time, did not differ from that of the other cases with the classic Thomsen form. Electromyography would thus seem to be a sure means of differentiating recessive from dominant forms of congenital myotonia in sporadic cases.